Hi,
The M.2 got Initialized as GPT by the clean install of Windows 10 with the machine and installation media setup for UEFI Bios.. Windows does this automatically as part of the installation, or if you do a Custom Install and select the Unallocated drive.. So that issue is resolved.. That is why I suggested the Clean Install... As a way to Initialize the drive properly with the hope that you could then proceed with the cloning\imaging..
That was before the issue of both drives not being usable at the same time popped up..
It is normally a piece of cake to add\install a new or additional HDD\SSD to Windows 10... Just plug it in..
The fact you are using a M.2, not a standard drive has complicated things more than just adding a 2nd SSD would...
I just took a quick look at the specs for your motherboard and it seems that SATA Ports 5+6 are disabled when you hook up a M.2 drive to the M2_1 connection.. I'm guessing that your SSD is plugged into 5 or 6..
Try plugging it in to one of the connections on the other end of the block of SATA Ports and see what happens..
BTW... Copying files from the SSD to the M.2 will not move the existing operating system to the M.2, or any programs.. When this is all fixed, you will need to decide if you still want to clone\image and keep all of your programs intact, or just start fresh with the clean install on the M.2..